In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrostatographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrostatographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the latent electrostatic image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner particles should be maintained within an adequate range.
Many well-known types of toner particles useful in dry developers comprise vinyl addition polymeric binder materials, chosen for their good combinations of advantageous properties, such as toughness, good adhesion to substrates, and fusing characteristics, such as the ability to be fixed to paper at relatively low fusing temperatures while not permanently adhering to fusing rolls, except at relatively high temperatures. As is well known, vinyl addition polymers useful as binder materials in toner particles can be linear, branched, or lightly crosslinked and can be fashioned from any of many different monomers, typically by free radical-initiated addition polymerization of monomers containing ethylenic unsaturation.
Also, toner particles in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature.
One general type of charge-control agent known to be useful in toner particles for dry developers comprises a quaternary phosphonium salt. A number of such quaternary phosphonium salt charge-control agents are described, for example, in U.S. Pat. Nos. 4,496,643 and 4,537,848. Unfortunately, many of those known charge-control agents can exhibit a number of drawbacks in some developers.
For example, some of the known quaternary phosphonium salt charge agents lack thermal stability and, thus, totally or partially decompose during attempts to mix them with known toner binder materials in well-known processes of preparing toners by mixing addenda with molten toner binders. Such processes are often referred to as melt-blending or melt-compounding processes and are commonly carried out at elevated temperatures. Thus, charge agents that are thermally unstable at temperatures encountered during melt-compounding can exhibit this decomposition problem.
Also, some of the known quaternary phosphonium salt charge-control agents have relatively high melting points. During melt-blending, a molten charge agent can be more quickly, efficiently, and uniformly dispersed in the molten toner binder than can a solid charge agent. Non-uniform dispersion can result in poor or inconsistent charge-control performance from toner particle to toner particle (among other undesirable effects discussed below). Therefore, it is a drawback to have a charge agent that will not become molten at the temperatures that will be encountered in melt-compounding, because such a charge agent will be slowly, inefficiently, and non-uniformly dispersed in the toner binder during some melt-blending processes.
Furthermore, some of the known quaternary phosphonium salt charge agents have relatively high electrical conductivity, which can lead to poor performance of some developers.
Also, some known quaternary phosphonium salt charge agents exhibit high sensitivity to changes in environmental relative humidity and/or temperature, which can lead to erratic performance of the charge agents under changing environmental conditions.
Additionally, some of the known quaternary phosphonium salt charge agents will adversely interact chemically and/or physically with other developer or copier components. For example, some will interact with carrier or carrier coating materials (e.g., fluorohydrocarbon polymer coatings such as poly(vinylidene fluoride)) and lead to premature carrier aging and shortened useful developer life. Some will interact with certain toner colorants to cause unacceptable hue shifts in the toner. Some will interact with copier fuser rollers (e.g., rollers coated with fluorohydrocarbon polymers such as poly(vinylidene fluoride-co-hexafluoropropylene)) to cause premature failure of the copier's toner fusing system. Some will interact with surface layers of electrostatographic elements to cause poor latent image formation and shortened useful element life.
Also, poor dispersibility of some of the known quaternary ammonium salt charge agents in some of the known vinyl addition polymeric toner binder materials, either because the charge agent remains solid during melt-compounding (as discussed above) or undergoes phase separation from the toner binder when it is attempted to increase its concentration therein, or because it is incompatible with or otherwise poorly dispersible in the binder, can lead to worsening of some of the problems mentioned above. Non-uniform dispersion of charge agent means that higher concentrations or agglomerations of charge agent will exist in some portions of the toner binder mix, compared to others. In typical melt-blending processes, the toner mixture is cooled and ground down to desired particle size after melt-blending. Agglomerations of charge agent provide sites in the mixture where fracture is more likely to occur during grinding. The new surfaces created by such fracture will have a higher concentration of charge agent than will internal sites. Thus, the final toner particles will have a higher surface concentration of charge agent than internal concentration. It should be readily appreciated that if a charge agent tends to adversely interact with the environment, copier components, or other developer components, higher surface concentrations of charge agent on the toner particles will lead to a greater degree of such interaction, thus exacerbating problems such as high conductivity, high environmental sensitivity, and premature failure of carrier and copier component materials.
Furthermore, in the known dry developers containing known quaternary phosphonium salt charge-control agents in toner particles, the charge-control agents are not chemically bonded to the other toner components, e.g., the polymeric binders. Therefore, the charge-control agents can migrate within, and exude from, the toner particles over time, causing non-uniform dispersion, inconsistent and changing toner charge, and worsening of adverse interactions noted above.
Also, many of the known quaternary phosphonium salt charge-control agents are not as efficient at creating the desired charge level as is desirable; i.e., it requires a relatively high concentration of the charge-control agent to produce the desired charge level. The greater the concentration of charge-control agent required, the greater will be the chance of phase separation and non-uniform dispersion, the greater will be the chance of adverse interactions and poor or inconsistent charging, and the greater will be the cost of the charge-control agent required per unit amount of toner.
Additionally, with many of the known quaternary phosphonium salt charge-control agents, the ability to produce changes in charge level by increasing charge-control agent concentration, falls off relatively quickly; i.e., a plateau of charge level versus concentration is reached, and this may occur before the charge reaches the desired level. In that case, no matter how much more the concentration of charge-control agent is increased, the desired charge level will never be achieved.
Also, the falling off of changes in charge level versus concentration with non-polymeric phosphonium salt charge agents is usually a gradual one; i.e., with those charge agents the charge level generally changes non-linearly with changes in concentration. This makes it difficult or impossible to determine what concentration of charge agent will be needed to produce a given charge level, except through a lengthy process of trial and error.
It would, therefore, be desirable to provide dry developers containing vinyl addition polymeric toner particles containing quaternary phosphonium salt compositions that perform the charge-controlling function well therein, while avoiding or minimizing all of the drawbacks noted above. The present invention does this.